Rigged model deforms on one side and doesent on the other
Switch to Solid mode. Select your armature and in the Properties panel > Object > Viewport Visibility, enable In Front. Select your mesh and in the Properties panel > Modifiers, choose the metarig as Object of the Armature modifier. Select the armature in Object mode, then shift select the mesh, switch to Weight Paint mode. Select the right upper arm bone, you can see that it influences the forearm mesh, same thing for the forearm bone which affects the upperarm mesh, they should not. So in the 3D View N panel > Tool tab > Brush, choose a Weight of zero or a Blend > Subtract, in the Options disable Front Faces Only and enable 2D Falloff, begin to paint until it looks like the left arm.